This invention relates to the manufacture of melt-blown or spun-bonded fabrics and combinations thereof with other natural or synthetic fibers such as wood pulp, cotton, hemp, rayon, polyester, nylon, or the like.
Melt-blown fabrics are relatively new in the art, and although they offer the advantage of extremely low cost, they have not achieved widespread acceptance as yet due to their relative stiffness and lack of drapability. Melt-blown fabrics are usually manufactured by blowing a line of molten polypropylene, polyethylene, or other synthetic filaments downwardly onto a moving condensing surface with sufficient force to randomly entangle the molten filaments just before they harden. The velocity of the air stream at the melt-blowing spinnerets can be controlled to produce a web made of fibers having a finite length or continuous filaments or combinations of both. The word "strand" as used hereinafter in the specification and claims of this application is hereby defined to mean fibers or filaments or both. A melt-blown web can also be deposited on one or both sides of a moving layer of wood pulp, cotton or rayon linters, or similar absorbent fibers which are temporarily held in place by suction to keep the fibers from scattering under the melt-blowing spinnerets. A binder can be added later to help hold the pulp layer together.
The melt-blown fabric produced by the above-described process is an entangled, partially-fused network of synthetic strands that are relatively brittle and are relatively weak because of their undrawn condition. There are several prior art methods of drawing webs by gripping the edges of the web and pulling the gripped edges apart, e.g. through the use of diverging belts or tentering frames, but these methods are not applicable to melt-blown webs because such webs are relatively non-uniform, i.e. they contain areas where the web is thinner and less dense than it is in the thicker places. Therefore, gripping the web at the edges and stretching the web tends to stretch the thin areas and leave the thick areas unchanged. This increases the nonuniformity of the web and makes it more susceptible to tearing. Also this type of stretching does nothing to improve the softness, bulk, drapability, and texture, which are all quite poor in melt-blown or spun-bonded fabrics and tend to restrict the use of these fabrics and to impede their acceptance.
Embossing webs using toothed rollers is not related to the invention described herein. Embossing constitutes compression deformation or crushing rather than tensile deformation. Furthermore, embossing doesn't orient molecules or increase strength as does the draw process of this invention.